Gas Turbines utilized in Marine and Industrial applications, especially Mechanical Drive applications, feature combustors as components and are often operated for extended periods of time at partial power. Partial power herein means operation at less than 100% load. As fuel prices increase, improved partial power efficiency is an attribute that is very much desired by operators.
Disposed within a turbine combustor are nozzles that serve to introduce fuel into a stream of air passing through the combustor. Igniters are typically used to cause a resulting air-fuel mixture to burn within the combustor. The burned air-fuel mixture is routed out of the combustor and on through a turbine or turbines to extract power which drives the compression system and provides useful work to an operator.
Dry-Low-Emissions (hereinafter, DLE) combustors are gas turbine engine components relying on lean premixed combustion that operate within bulk flame temperature (hereinafter, Tflame) windows where emissions are within limits. Tflame is the adiabatic flame temperature calculated to result from complete combustion of air and fuel entering fueled combustor cups. At a maximum value for Tflame, the emissions of oxides of Nitrogen (NOx) increases sharply. At a minimum value for Tflame, (hereinafter, Tflame min), the emission of Carbon Monoxide (CO) as an undesirable by-product of combustion increases. In the art, typical operation is to bleed compressor air overboard in order to lower this undesirable emissions by-product. However, such prior art use of overboard bleed air extraction serves to maintain local Tflame in a desired narrow band of temperature range but it also decreases partial power efficiency, thereby increasing fuel operating expenses.
Therefore a problem to be solved is to maximize the partial power efficiency characteristics of DLE gas turbines while minimizing undesirable emissions by-products. Overboard bleed air extraction is typically used at part power operation to maintain acceptable emissions in a DLE system by holding combustor bulk flame temperature in a narrow band. In addition, the prior art has seen a limited amount of staging of premixed rings and cups. As emissions regulations become more stringent, the acceptable window of bulk flame temperatures is growing much more narrow and difficult to achieve. As the Tflame bands narrow, the engine requires increased use of bleed air to remain in the window of acceptable bulk flame temperatures.
Bleed Avoidance Technology (BAT) pertains to a method to improve partial power efficiency in Dry-Low-Emissions (DLE) engines by reducing the amount of bleed air extraction. Embodiments are provided that include BAT to enable diffusion flame operation at low power conditions, premixed flame operation at high power conditions, and a combination of premixed/diffusion flame operation at intermediate power settings thereby providing a means to reduce bleed air requirements to improve performance while simultaneously meeting stringent emissions requirements. Enhanced Lean Blowout (hereinafter, ELBO) refers to the concept that selected features allow for operation at lean air/fuel ratios very close to air/fuel ratios and temperatures seen as at the edge of where existing systems might suffer a loss of flame entirely—“blowout.” Variable ELBO refers to ability to vary fuel delivery as desired in such a manner as to optimize lean operation.
Fuel system design requirements in prior art DLE engines have concentrated primarily on full load efficiency and emissions. While a worthwhile goal and one that begins to meet ever-increasing needs in the Art, embodiments utilizing variable ELBO fuel provide enhanced efficiency and reduced emissions at a far wider range of power settings from start-up to full power. Alternatives provide variable ELBO to a majority of the premixes to enhance fuel system functionality and to optimize the reduction of full-power emissions and achieve a partial power turndown in Tflame.
To improve partial power efficiency in legacy DLE applications, the primary approach has been to add circumferential staging modes wherein several cups of the combustor are turned off (i.e not fueled). This approach introduces localized cold zones in the combustor, thereby increasing CO emissions and requiring additional control valves and additional time to map the circumferential modes.
Designs in the Art include the use of two-cup and three-cup premixers. Illustrations provide for an A cup, a B cup, and a C cup for those systems utilizing three cups in the premixer. Other designs in the Art to reduce the need for bleed air extraction include Variable Area Turbine Nozzles (VATN) and bleed re-injection (also known as bypass bleed) back into the power turbine. However, these prior art designs are comparatively expensive, have experienced limited reliability, and are technically complex compared to the present embodiments.
In further detail, prior art DLE engines extract compressor bleed to provide overboard bleed air extraction as a means to maintain combustor flame temperatures above a lower threshold below which CO and UHC emissions increase rapidly. The lower threshold value is referred to as incipient lean blow out.
Solution
In contrast, embodiments are provided that provide a means to forestall incipient lean blow out by improving flame stabilization thereby enabling the combustor to operate with acceptable emissions at lower flame temperature. Embodiments allow the combustor to operate at lower bulk flame temperatures during partial power operation, thereby reducing or even eliminating the usage of inefficient overboard bleed air extraction.
In solving the problem, embodiments are provided that utilize variable ELBO as a feature of the premixer and that inject fuel directly into a combustion chamber. This use of ELBO fuel improves flame stabilization by creating small high temperature diffusion flames that serve as ignition sources for the fuel-air mixture entering the combustor through one or more premixers. In contrast, most of the combustion is lean premixed. The one or more premixers may each have one or more cups with embodiments including those with two cups, A and B (as shown in FIG. 1); and alternatives including those with three cups, A, B and C (not shown). Embodiments and alternatives are provided that increase the range of flame temperatures (Tflame) that allow desired efficient operation at or under acceptable emissions levels. The solution includes the use of variable and independently controlled ELBO fuel thereby allowing optimization of emissions throughout the operating range and the provision of a control system featuring control/staging logic to allow for a flame to be primarily diffusion flame in operation at low power conditions and primarily premixed operation at high power conditions. Operators clearly recognize the cost savings associated with just one percentage point improvement in partial power thermal efficiency. Therefore, these embodiments are of high value to all operators in that measurable results from use of the embodiments provided include an improvement of up to 3 percentage points in partial power thermal efficiency when compared to known art DLE gas turbines operating under similar conditions. While increasing partial power efficiency, embodiments also reduce fuel system cost and complexity. Additional alternatives utilize diffusion flame and thereby reduce combustion acoustics. As such, embodiments serve to improve combustion system durability by reducing transient acoustics. Compared to the Art of staged DLE combustors, embodiments also provide the ability to maintain a more consistent exit profile and pattern factor as well as a lower turbine inlet temperature during partial power operation. This leads to improved hot section durability, sensor accuracy in measuring exhaust temperatures and reliability of the entire system. In general, diffusion fuel flow allows for good operability. Premixed fuel flow allows for good emissions characteristics. Combined diffusion and premixed fuel flow allow for an optimization of both operability and emissions.